The present invention relates generally to the reduction of noise in digital audio systems and particularly to digital audio systems employing adaptive differential coding techniques.
In this application, the terms "low frequency" and "high frequency" refer to the audible range and are used interchangeably with the terms "lower portion of the audio spectrum" and "upper portion of the audio spectrum," respectively. For the purposes of comprehension, low frequencies may be taken as those below about 1 kHz, and high, those above about 2 kHz. In practical embodiments different boundaries may apply depending, for example, on the upper and lower frequency limits of the system, the nature of the audio signals carried by the system (e.g. music, speech), etc.
Differential digital coding techniques, chiefly differential pulse code modulation (PCM) using one or more bits to represent a change from one or more previous quantum levels (rather than an absolute value relative to a reference level) are well known. Single bit differential PCM systems are generally referred to as delta modulation. All forms of differential PCM, including delta modulation, that employ fixed quantum step sizes suffer from the problem that if the waveform of the signal applied to the encoder changes too rapidly, the quantizer cannot keep pace. See generally PCM and Digital Transmission Systems, by Frank F.E. Owen, McGrawHill Book Company, San Francisco, 1982, pages 87-90.
One known solution is to provide for variable rather than fixed quantum step sizes such that the size of the quantizing steps varies with the time differential (slope) of the input signal, thus allowing the system to track more closely a rapidly varying input signal. Such systems are referred to as adaptive differential digital coding systems and include adaptive differential PCM (ADPCM) and adaptive delta modulation (ADM). Examples of a type of adaptive delta modulation system referred to as continuously variable slope delta modulation (CVSD) are set forth in U.S. Pat. Nos. 4,190,801 and 4,305,050, which are incorporated herein, each in their entirety, by reference.
A consequence of the fact that in adaptive differential digital coding systems the size of the quantizing steps varies with the slope of the input signal is that the quantizing error or noise depends on the input signal, being least for low slope signals and increasing for higher slope signals. The spectrum of the quantizing noise extends across the whole of the audio bandwidth, and typically has a roughly uniform power spectral density (that is, it resembles white noise).
It is a property of the human ear that low level noise in the same region of the spectrum as a loud sound cannot be perceived, an effect known as masking. However noise in parts of the spectrum remote from a loud or dominant signal remains audible. Masking permits the design of complementary noise reduction systems (companders) with which modulation of the background noise by the program is imperceptible. Since varying noise levels are much more obtrusive than constant ones, absence of audible noise modulation is an essential property of high quality audio systems for the reproduction of music.
In companders operating in analog audio systems the audible effects of noise modulation can be reduced by band-splitting or sliding band techniques whereby the degradation of signal to noise ratio accompanying a particular signal is confined to the same area of the spectrum as the signal, leaving the noise levels in other parts of the spectrum unaffected. Consequently, noise modulation only occurs in areas of the spectrum where it is masked by the dominant signal controlling the companding and is not perceived by the listener. Examples of band-splitting analog companders are given in U.S. Pat. Nos. 3,846,719, 3,903,485 and Journal of the Audio Engineering Society, Vol. 15, No. 4, October, 1967, pp. 383-388. Both of said U.S. patents are incorporated herein by reference, each in their entirety. Analog companders employing sliding band techniques are described in U.S. Pat. No. Re 28,426, U.S. Pat. Nos. 3,757,254, 4,072,914, 3,934,190 and Japanese Patent Application No. 55529/71. All four of said U.S. patents are incorporated herewith by reference, each in its entirety.
The adapting function in adaptive differential digital coding systems is a form of companding or noise reduction, inherently giving rise to modulation of the wideband quantizing noise by the signal. The slope of an input signal is proportional to the product of its frequency and amplitude. When the input signal contains predominantly high frequencies, its slope is high and the quantizing noise increases. The high frequency noise components will be masked by the signal, but the unmasked changes in the low frequency noise components will often be audible. If a wideband analog noise reduction system is used in conjunction with an adaptive differential digital system, the low frequency noise is increased even further by the expansion process. Consequently, there is a need to reduce such unmasked changes in audible low frequency noise components particularly in high quality audio systems for the reproduction of music, but such unmasked changes in low frequency noise components cannot be reduced by noise reduction companding.
When the input signal contains predominantly low frequencies, its slope is low and the quantizing noise remains low. Any variation in the low frequency components of the quantizing noise will be masked by the low frequency signals, and variations in the high frequency components, which may not be masked, may be insignificant because of the low level of the noise, or, if not insignificant there may also be a need to reduce such audible high frequency components in order to achieve noise reduction over the entire audio spectrum. In certain systems high frequency noise may be significant because of the use of techniques to shift the noise spectrum.